This invention is directed to the field of fault detectors, more particularly to a fault detector system incorporating a fiber optic fault detection mechanism to alert a user thereof to potential problems, such as mechanical damage or overheating conditions to the electrical conductors or powered devices.
The present invention relates to a mechanical fault detector system where fiber optics represent the fault detecting mechanism for the system. Fiber optics, as a signal transmission medium, is a recent innovation compared to electrical and other types of conductors. Fiber optics offer distinct advantages to the more conventional route as one can use a single filament or bundle a series of fiber optic filaments in a single cable, transmission losses are significantly reduced, and internal heat generation is not a problem.
A fiber optic system operates by the use of a laser to produce a light beam, a modulator which modulates the light beam before it is fired along the glass or fiber optic cable, to finally a light detector and amplifier.
While a fiber optic system offers distinct advantages over a convention electrical conductor system, there are limitations which must be noted. There are, for example, minimum bend radius requirements for fiber optic cables. For fiber optic cables not in tension, the minimum bend radius is 10xc3x97diameter; cables loaded in tension may not be bent at less than 20xc3x97diameter.
As an alternative to the traditional glass optic fibers, a recent development is a plastic fiber optic. However, with such alternative, there may be temperature limitations which must be considered. An optimum temperature range may be xe2x88x9230 to +70 degrees C. Operating outside the range may cause embrittlement of the plastic, or transmission loss and fiber shrinkage. Notwithstanding such limitations, considerable benefits may be gained through the use of fiber optic signal transmission systems.
A primary object of this invention is to detect mechanical or potential mechanical damage to an electrical conductor system or equipment, by the use of a fiber optic fault detector mechanism. The prior art offers different types of temperature or damage alerting devices for electrical conductor systems.
Historically, electrical apparatus have depended on over-current devices such as circuit breakers and fuses to protect the apparatus from over temperature conditions which might lead to equipment damage or personnel injury resulting from fire or smoke. Overcurrent devices suffer from limitations in their effectiveness, in that over temperature conditions can result from normal current flow through equipment suffering from damaged conductors or poor electrical connections. For example, an electrical cord containing stranded wire conductors may overheat if some of the strands are broken, even if less than rated current flows through the cord. Many fires have been caused due to poor connections in plugs, receptacles, and connection strips in cords and appliances.
Ground fault interrupters (GFI) are another common safety device used in electrical equipment. While such devices are effective in reducing electrical shock and equipment damage to shorts to ground, they are ineffective in cases where the fault does not result in current flow to ground. For example, a GFI will not prevent a fire in the case of broken conductor strands or poor connections because no ground current flow occurs. Neither GIF""s nor over current devices protect a cord or device from over temperature conditions resulting from external sources of heat such as excessive ambient temperature conditions or contact with hot burners, hot piping, etc. Neither device may be effective if the cord is unable to dissipate normal heat. For example, the cord or appliance may become covered with thermally insulating material that prevents dissipation of heat due to normal current flow.
Temperature sensors such as resistance temperature sensors (Rtes) and thermistors are commonly used to detect over temperature conditions in equipment. These devices may be used to initiate alarms or relays to interrupt current to the device upon over temperature. While these devices are effective in sensing temperature in a small or enclosed space, they are ineffective in sensing over temperature over a long distance or in a large volume unless many devices are employed. Use of sufficient devices for sensing temperature over a long distance or large volume raises the cost of protection substantially. Also, the wiring needed for connecting a large number of sensors complicates the device and increases the size and bulk of the device.
U.S. Pat. No. 5,841,617 represents one significant approach to an alternative for a fault detection device for a conventional electrical conductor system. The patent teaches an electrical safety device comprising a sensor strip disposed in the insulation of an electrical power cord or other electrical apparatus. The sensor strip is made of a conductive polymer with a positive temperature coefficient of resistivity which increases with temperature. The sensor strip forms a series connected loop connected to an impedance measuring circuit. A relay interrupts current to the conductors of the cord when the impedance of the sensor strip increases due to an over temperature condition. The sensor strip is positioned between the conductors of the cord and the outside surface of the cord. The position of the sensor strip allows the strip to act as a mechanical damage sensor, opening the series connected loop before an energized conductor is exposed.
One proposal offered in the prior art, and directed to a fiber optic sensor, is taught in U.S. Pat. No. 4,488,040. The patent is directed to a fiber optic sensor useful in intrusion detection systems for sensing seismic or pressure disturbances provided by an intruder. The invention, thereof comprises a cable having an optical fiber core for transmitting the output of a laser, a hard electrically nonconductive strand wound helically on and coextensive with the fiber, and a compliant tubular sleeve over the fiber and strand. The coils of the strand are axially spaced apart and circumferentially engage the fiber. The sleeve fits snugly over the strand and fiber and transmits through the strand coils to the fiber forces resulting from disturbances to be detected, subjecting the fiber to bending stresses. With cable connected at one end to the laser output, the polarization of light transmitted by the fiber is changed by the induced stresses and is detected at the other end of the cable to indicate the occurrence of the disturbance.
Neither of the above prior art patents offers the answers to providing a fault detection system utilizing a fiber optic fault detecting mechanism. The manner by which the present invention provides such answers will become apparent, particularly to those persons skilled in the art, from the following description.
The present invention is directed to a fault detection system incorporating a fiber optic fault detecting mechanism to protect an electrical conductor, where the fault may be mechanical damage to the insulation. More particularly, the invention is directed to apparatus for detecting mechanical damage in a wire or cable by disposing an optical fiber sensor between an energized electrical conductor in the cable and the outside surface of the wire or cable. Mechanical damage, such as fraying, cutting or abrasion, will damage or sever the fiber optical fiber sensor before the electrical conductor is exposed or contacted. An optical signal measuring device, such as an optical time domain reflectometer (OTDR), coupled to the optical fiber sensor by an optical coupler, detects the defect in the optical fiber sensor caused by mechanical damage and provides an alarm or control action. The OTDR will also indicate the location of the damage to speed analysis and repair.
Accordingly, an object of this invention is to provide a fault detection system utilizing a fiber optic fault detecting mechanism to protect an electrical conductor, where the fault to be detected will be manifested by cutting or severing the fiber optic sensor, excessive pressure on the fiber optic sensor, or unduly high temperatures.
Another object hereof is the provision of plural fault sensing fiber optic elements to quickly determine the nature of the fault.
These and other objects of the invention will become apparent to those skilled in the art from the following specification, particularly when read in conjunction with the accompanying drawings.